Electrical capacitors are used to provide a start up voltage for a variety of electrical and electronic devices including household appliances, heating and air conditioning systems and power supplies for industrial uses.
A capacitor is generally formed by a housing such as a cylindrical shaped container made of aluminum having therein one or more capacitor rolls. The capacitor rolls are typically made of wound layers of an electrically inert or insulative material such as plastic (e.g. polypopylene) or paper having thereon a very thin layer of an electrically conductive material such as vaporized metal (e.g. zinc). The ends of the capacitor rolls are coated with a metallized layer (e.g. zinc). The metallized end is electrically connected to terminals to provide a pathway for the flow of an electric charge stored in the capacitor rolls to an electrical or electronic device connected to the terminals. Capacitors used for low output requirements do not have a container as such but instead employ an encapsulating layer of a synthetic material such as epoxy to encapsulate or house the components of the capacitor.
The capacitor may optionally be provided with a dieletric fluid (e.g. polypropylene glycol) which serves to suppress partial discharges in areas of high electrical stress and also functions as a heat transfer medium. Dielectric fluid containing capacitors are known as "wet" capacitors and capacitors which do not contain dielectric fluid are known as "dry" capacitors. The design requirements of a capacitor are in part dependent on the "capacitance" which is a measure of how much voltage the capacitor will retain which is typically in the range of 5 to 50 microfarads for many electrical and/or electronic applications.
Loss of capacitance leads to decreased efficiency and eventual breakdown of the capacitor. The leading cause of loss of capacitance is corrosion caused by the presence of contaminants which are primarily polar compounds and molecules present in the capacitor. These contaminants include, for example, water, carbon dioxide, ammonia, chlorine gas, sulfur dioxide and others. Contaminants may be present in the capacitor at the time of manufacture, as for example, the presence of moisture in the housing and from soldering operations used to electrically connect the capacitor rolls to the terminals.
Such contaminants tend to corrode the thin metal layers of the capacitor rolls thereby reducing the surface area of the metallized layers. Since the amount of capacitance is directly proportional to the surface area of the metallized layers, a resultant reduction in surface area leads to a reduction in capacitance.
Another factor in rating the quality of a capacitor is the so called "dissipation factor" which is a measure of the percentage of electrical energy lost by the capacitor over time.
All capacitators tend to lose some amount of electrical energy. In some cases the lost electrical energy is transformed into heat energy which is evidenced by a heat rise within the capacitator. The lower the dissipation factor, the longer the life span of a capacitor. In addition, the dissipation factor tends to increase over time since lost electrical energy is often caused by the same factors which give rise to a loss of capacitance (i.e. corrosion caused by the presence of contaminants especially polar compounds and molecules.
Heretofore efforts at reducing corrosion within the capacitor housing have focused on increasing the efficiency of sealing the cover having thereon electrical terminals to the container which contains the capacitor rolls. Better seals and more rapid and efficient sealing processes, however, have not been successful in eliminating the corrosive effect of contaminants contained within the sealed capacitor. As a result, present day capacitors have a limited useful life.
It is therefore an object of the invention to provide an electrical capacitor which has a significantly longer useful life than present capacitors.
It is a further object of the invention to provide an electrical capacitor which includes therein an adsorbent material capable of adsorbing contaminants thereby reducing corrosion within the capacitor.
It is a still further object of the invention to provide an electrical capacitor which is highly resistant to a loss of capacitance and has a significantly lower dissipation factor.